Page:Popular Science Monthly Volume 67.djvu/67

Rh of the fittest.' He constantly compares the action of natural selection to that of artificial selection, in which he supposes that the breeder picks out those individual differences of the kind known as 'fluctuating variations.'

Modern zoologists who claim that the Darwinian theory is sufficiently broad to include the idea of the survival of definite variations seem inclined to forget that Darwin examined this possibility and rejected it. The grounds for this rejection seemed valid at the time, but a wider knowledge of the facts has shown that the problem is simpler than Darwin was aware of.

While Darwin uses the term 'struggle for existence' in a very loose, and often in a metaphorical sense, as he himself points out; and while it is true that he speaks of varieties and even species struggling with each other, yet the central idea is that natural selection adapts the organism to its environment by picking out and accumulating slight, individual differences. It is, indeed, only in this way that natural selection appears in the rôle of a creative factor in evolution, and it is this power to build up new adapted types that appears to give the theory its high value.

Numerous cases of discontinuous variation have been known for a long time, so that it is no mere assumption that such occur in nature. Darwin himself has collected many instances of this sort, and amongst domesticated animals and plants sudden variations have been frequently recorded. In fact there can be no doubt, especially in the case of plants, that such variations have often been utilized by the breeder, even unconsciously at times. A few cases of this sort have been described even for domesticated pigeons, and it is not improbable that the great variety of domesticated breeds may, in part, have arisen in this way, and not as the result of the selection of the individual fluctuations of the wild rock-pigeon. Darwin argued that sudden variations can not have been the source of new species, because, as a rule, when they are crossed with the parent form the offspring are not intermediate, but are exactly like one or the other parent, and it is a well known fact that when wild species are crossed the hybrid is midway in character between its parents. Therefore, Darwin believed, wild species could not have arisen as discontinuous variations.

Our knowledge on these points has greatly increased since Darwin's time, especially in recent years, mainly as the result of de Vries's work on the evening primrose. It appears that there are several different kinds of definite variations. The simplest cases are those in which some one character suddenly becomes lost, as when an albino mouse arises from a gray mouse. If such an albino is crossed with the parent gray form the offspring in the first generation are all like the gray mouse, but if these offspring are then inbred, they will produce some